1. Field of the Inventions
The present invention relates to a phase-change memory for storing information by utilizing a reversible phase change which may occur between a crystalline phase and an amorphous phase, a writing apparatus for writing information in the memory, a reading apparatus for reading information from the memory, and writing/reading methods therefor.
2. Description of the Related Art
A phase-change memory in which information can be, recorded or erased by applying electric energy such as an electric current is known. The material used as a recording layer of such a phase-change memory causes reversible change between the crystalline phase and the amorphous phase due to increases in temperature which results from the application of the electric energy. Generally, the electric resistance of the crystalline phase is low, whereas the electric resistance of the amorphous phase is high. The phase-change memory is a non-volatile memory in which binary information is recorded by utilizing the difference in electric resistance between the crystalline phase and the amorphous phase.
In recent years, along with the increase in amount of information to be recorded in a memory, a memory having a larger capacity has been demanded. In order to increase the capacity of a phase-change memory, two suggestions have been provided: (1) the area of a memory cell for recording a binary value a reduced, and a plurality of such memory; cells are arranged in a matrix (increase in surface density) (2) information of a multi-value is stored in a single memory cell. In this specification, the xe2x80x9cmulti-valuexe2x80x9d does not include the xe2x80x9cbinary valuexe2x80x9d.
Regarding suggestion (1) since there is a limit to a miniaturization process in a production technique such as photolithography, there is also a limit to the increase in surface density. Thus, it is impossible to drastically increase the capacity of a phase-change memory.
A known conventional technique for recording multi-value information in a single memory cell is disclosed in Japanese National Phase PCT Laid-Open Publication No. 11-510317. According to this conventional technique, the resistance value of a recording layer of a memory cell is controlled in a stepwise manner, whereby multi-value information can be stored in the memory cell. However, such a stepwise control of the phase state in a single recording layer involves greater difficulty rather than control of the phase state between the two phase states, i.e., the crystalline phase and the amorphous phase.
According to one aspect of the present invention, a memory includes: a first recording layer for recording information by utilizing a reversible phase change between a crystalline phase and an amorphous phase which occurs due to increases in temperature caused by application of an electric current pulse; and a second recording layer for recording information by utilizing a reversible phase change between a crystalline phase and an amorphous phase which occurs due to increases in temperature caused by application of an electric current pulse, wherein the crystallization temperature of the first recording layer, Tx1, and the crystallization temperature of the second recording layer, Tx2, have the relationship Tx1 less than Tx2, the crystallization time of the first recording layer, tx1 and the crystallization time of the second recording layer, tx2, have the relationship tx1 greater than tx2, and Ra1+Ra2, Ra1+Rc2, Rc1+Ra2, and Rc1+Rc2 are different from one another where the resistance value of the first recording layer In the amorphous phase is Ra1 the resistance value of the first recording layer in the crystalline phase is Rc1 the resistance value of the second recording layer in the amorphous phase is Ra2, and the resistance value of the second recording layer in the crystalline phase is Rc2.
In one embodiment of the present invention, the melting point of the first recording layer, Tm1, satisfies the relationship 400xe2x89xa6Tm1(xc2x0 C.)xe2x89xa6800.
In another embodiment of the present invention, the melting point of the second recording layer, Tm2, satisfies the relationship 300xe2x89xa6Tm2(xc2x0 C.) xe2x89xa6700.
In still another embodiment of the present invention, the crystallization temperature of the first recording layer, Tx1, satisfies the relationship 130xe2x89xa6Tx1,(xc2x0 C.)xe2x89xa6230.
In still another embodiment of the present invention, the crystallization temperature of the second recording layer, Tx2, satisfies the relationship 160xe2x89xa6Tx2(xc2x0 C.)xe2x89xa6260.
In still another embodiment of the present invention, the crystallization time of the first recording layer, tx1, satisfies the relationship 5tx1(ns)xe2x89xa6200.
In still another embodiment of the present invention, the crystallization time of the second recording layer, tx2, satisfies the relationship 2xe2x89xa6txx2(ns)xe2x89xa6150.
In still another embodiment of the present invention, the first recording layer includes three elements, Ge, Sb, and Te; and the second recording layer includes (Sb-Te)-Ml, where Ml is at least one selected from a group consisting of Ag, In, Ge, Sn, Se, Bi, Au, and Mn.
In still another embodiment of the present invention, the first recording layer is formed on a substrate, and the upper electrode is formed on the second recording layer.
In still another embodiment of the present invention, a lower electrode is formed between the substrate and the first recording layer.
In still another embodiment of the present invention, an intermediate layer is formed between the first recording layer and the second recording layer.
In still another embodiment of the present invention, the specific resistance ra1, of the first recording layer in the amorphous phase is 1.0xe2x89xa6ra1(xcexa9xc2x7cm)xe2x89xa61xc3x971xc3x97107.
In still another embodiment of the present invention, the specific resistance ra2, of the second recording layer in the amorphous phase is 2.0xe2x89xa6ra2(xcexa9xc2x7cm)xe2x89xa62xc3x97107.
In still another embodiment of the present invention, the specific resistance rc1 of the first recording layer in the crystalline phase is 1xc3x9710xe2x88x923xe2x89xa6rc1(xcexa9xc2x7cm)xe2x89xa61.0.
In still another embodiment of the present invention, the specific resistance rc2 of the second recording layer in the crystalline phase is 1xc3x9710xe2x88x923xe2x89xa6rc2(xcexa9xc2x7cm)xe2x89xa61.0.
According to another aspect of the present invention, there is provided a writing apparatus for writing information in a memory, the memory including: a first recording layer for recording information by utilizing a reversible phase change between a crystalline phase and an amorphous phase which occurs due to increases in temperature caused by application of an electric current pulse; and a second, recording layer for recording information by utilizing a reversible phase change between a crystalline phase and an amorphous phase which occurs due to increases in temperature caused by application of an electric current pulse, wherein the crystallization temperature of the first recording layer, Tx1 and the crystallization temperature of the second recording layer, Tx2, have the relationship Tx1 less than Tx2, the crystallization time of the first recording layer, tx1 and the crystallization time of the second recording layer tx2, have the relationship tx1 greater than tx2, and Ra1+Ra2, Ra1+Rc2, and Rc1+Rc2 are different from one another where the resistance value of the first recording layer in the amorphous phase is Ra1, the resistance value of the first recording layer in the crystalline phase is Rc1, the resistance value of the second recording layer in the amorphous phase is Ra2, and the resistance value of the second recording layer in the crystalline phase is Rc2, and the writing apparatus including: a pulse generator for generating at least first to third electric current pulses and an application section through which the at least first to third electric current pulses are applied to the first recording layer and the second recording layer, wherein in order to change the first recording layer from the amorphous phase to the crystalline phase while the phase state of the second recording layer is kept unchanged, the pulse generator generates the first electric current pulse which provides a temperature (T) that satisfies Tx1xe2x89xa6T less than Tx2 during a time (t) that satisfies tx1xe2x89xa6t, in order to change the second recording layer from the amorphous phase to the crystalline phase while the phase state of the first recording layer is kept unchanged, the pulse generator generates the second electric current pulse which provides a temperature (T) that satisfies Tx2xe2x89xa6T during a time (t) that satisfies tx2xe2x89xa6t less than tx1, and in order to change both the first recording layer and the second recording layer from the crystalline phase to the amorphous phase, the pulse generator generates the third electric current pulse which provides a temperature equal to or higher than the higher one of the melting points of the first and second recording layers.
In one embodiment of the present invention, the pulse amplitude of the first electric current pulse, Ic1, is 0.02xe2x89xa6Ic1(mA)xe2x89xa610, and the pulse width of the first electric current pulse, tc1, is 5xe2x89xa6tc1(ns)xe2x89xa6200.
In another embodiment of the present invention, the pulse amplitude of the second electric current pulse, Ic2, is 0.05xe2x89xa6Ic2(mA)xe2x89xa620, and the pulse width of the second electric current pulse, tc2, is 2xe2x89xa6tc2(ns)xe2x89xa6150.
In still another embodiment of the present invention, the pulse amplitude of the third electric current pulse, Ia1, is 0.1xe2x89xa6Ia1(mA)xe2x89xa6200, and the pulse width of the third electric current pulse, ta1, is 1xe2x89xa6ta1(ns)xe2x89xa6100.
In still another embodiment of the present invention, in order to change both the first recording layer and the second recording layer from the amorphous phase to the crystalline phase, the pulse generator generates a fourth electric current pulse which provides a temperature (T) that satisfies Tx2xe2x89xa6T during a time (t) that satisfies tx1xe2x89xa6t.
In still another embodiment of the present invention, the pulse amplitude of the fourth electric current pulse, Ic12, is 0.05xe2x89xa6Ic12(mA) xe2x89xa620, and the pulse width of the fourth electric current pulse, tc12, is 5xe2x89xa6tc12(ns)xe2x89xa6200.
In still another embodiment of the present invention, when the melting point of the first recording layer, Tm1, and the melting point of the second recording layer, Tm2, have the relationship Tm1 and Tm2, in order to change the recording layer having the lower one of the melting points Tm1 and Tm2 from the crystalline phase to the amorphous phase while the phase state of the recording layer having the higher one of the melting points Tm1 and Tm2 is kept at the crystalline phase, the pulse generator generates a fifth electric current pulse which provides a temperature equal to or higher than the lower one of the melting points Tm1 and Tm2 and lower then the higher one of the melting points Tm1 and Tm2.
In still another embodiment of the present invention, the pulse amplitude of the fifth electric current pulse, Ia2, is 0.05xe2x89xa6Ia2(mA)xe2x89xa6160, and the pulse width of the fifth electric current pulse, ta2, is 1xe2x89xa6ta2(ns)xe2x89xa6100.
According to still another aspect of the present invention, there is provided a reading apparatus for reading information from a memory, the memory including: a first recording layer for recording information by utilizing a reversible phase change between a crystalline phase and an amorphous phase which occurs due to increases in temperature caused by application of an electric current pulse; and a second recording layer for recording information by utilizing a reversible phase change between a crystalline phase and an amorphous phase which occurs due to increases in temperature caused by application of an electric current pulse, wherein the crystallization temperature of the first recording layer. Tx1, and the crystallization temperature of the second recording layer, Tx2, have the relationship Tx1 less than Tx2, the crystallization time of the first recording layer, tx1, and the crystallization time of the second recording layer, tx2, have the relationship tx1 greater than tx2, and Ra1+Ra2, Raa1+Rc2, Rc1+Ra2, and Rc1+Rc2 are different from one another where the resistance value of the first recording layer in the amorphous phase is Ra1 the resistance value of the first recording layer in the crystalline phase is Rc1, the resistance value of the second recording layer in the amorphous phase is Ra2, and the resistance value of the second recording layer in the crystalline phase is Rc2, and the reading apparatus including; an application section through which an electric current pulse is applied to the first and second recording layers; a resistance measurement device for measuring a sum of the resistances of the first and second recording layers; and a determination section for determining which of the four different sums of resistance values, Ra1+Ra2, Ra1+Rc2, Rc1+Ra2, and Rc1+Rc2, the measured sum of the resistance values of the first and second recording layers is equal to.
In one embodiment of the present invention, the electric current pulse has an amplitude Ir having a size such that a phase change is not caused in the first and second recording layers.
In another embodiment of the present invention, the amplitude Ir of the electric current pulse is Ir, (mA)xe2x89xa60.02.
According to still another aspect of the present invention, a memory includes N recording layers (N is a natural number which satisfies N greater than 2) for recording information by utilizing a reversible phase change between a crystalline phase and an amorphous phase which occurs due to increases in temperature caused by application of an electric current pulse, wherein the crystallization temperature Txm of the m-th recording layer (1xe2x89xa6mxe2x89xa6N) satisfies the relationship Tx1 less than Tx2 less than . . .  less than Txmxe2x88x921,  less than Txm less than Txm+1 less than . . .  less than Txn, the crystallization time txm of the m-th recording layer satisfies the relationship tx1 greater than tx2 greater than . . .  greater than txmxe2x88x921 greater than txm greater than txm+1 greater than . . .  greater than txN, and the resistance values of the N recording layers in the amorphous phase are different from one another, the resistance values of the N recording layers in the crystalline phase are different from one another, and the sum of the resistance values of the N recording layers is one of 2N values.
According to still another aspect of the present invention, there is provided a writing apparatus for writing information in a memory, the memory including N recording layers (N is a natural number which satisfies N greater than 2) for recording information by utilizing a reversible phase change between a crystalline phase and an amorphous phase Which occurs due to increases in temperature caused by application of an electric current pulse, wherein the crystallization temperature Txm of the m-th recording layer (1xe2x89xa6mxe2x89xa6N) satisfies the relationship Tx1 less than Tx2 less than . . .  less than Txmxe2x88x921 less than Txm less than Txm+1 less than  . . .  less than Txn, the crystallization time to of the m-th recording layer satisfies the relationship tx1 greater than tx2 greater than  . . .  greater than txmxe2x88x921 greater than txm greater than txm+1 greater than  . . . txN, and the resistance values of the N recording layers in the amorphous phase are different from one another, the resistance values of the N recording layers in the crystalline phase are different from one another, and the sum of the resistance values of the N recording layers is one of 2N values, and the writing apparatus including a pulse generator for generating at least N crystallization pulses and amorphization pulse, and an application section through which the at least N crystallization pulses and amorphization pulse are applied to the N recording layers, wherein in order to change only the m-th recording layer from the amorphous phase to this crystalline phase while the phase states of the other recording layers are kept unchanged, the pulse generator generates a crystallization pulse which provides a temperature (T) that satisfies Txmxe2x89xa6Tx less than Tx(m+1) during a time (t) that satisfies txmxe2x89xa6tx less than tx(mxe2x88x921), and in order to change all of the N recording layers from the crystalline phase to the amorphous phase, the pulse generator generates the amorphization pulse which provides a temperature equal to or higher than the highest one of the melting points of the N recording layers.
In one embodiment of the present invention, in order to change all of the N recording layers from the amorphous phase to the crystalline phase, the pulse generator generates an electric current pulse which provides a temperature (T) that satisfies TxNxe2x89xa6Tx, during a time (t) that satisfies tx1xe2x89xa6tx.
In another embodiment of the present invention, in order to change the m-th to (m+nxe2x88x921)th recording layers among the N recording layers from the amorphous phase to the crystalline phase, the pulse generator generates an electric current pulse which provides a temperature (T) that satisfies Tx(m+nxe2x88x921)xe2x89xa6Tx(m+n) during a time (t) that satisfies txmxe2x89xa6tx less than tx(mxe2x88x921).
In another embodiment of the present invention, when each of one or more recording layers among the N recording layers has a melting point equal to or lower than a temperature Tm, and each of the other recording layers among the N recording layers has a melting point higher than the temperature Tm, in order to change the one or more recording layers from the crystalline phase to the amorphous phase while the other recording layers are kept at the crystalline phase, the pulse generator generates an electric current pulse which produces the temperature Tm.
According to still another aspect of the present invention, there is provided a reading apparatus for reading information from a memory, the memory including N recording layers (N is a natural number which satisfies N greater than 2) for recording information by utilizing a reversible phase change between a crystalline phase and an amorphous phase which occurs due to increases in temperature caused by application of an electric current pulse, wherein the crystallization temperature Txm of the m-th recording layer (1xe2x89xa6mxe2x89xa6N) satisfies the relationship Tx1 less than Tx2 less than  . . .  less than Txmxe2x88x921 less than Txm less than Txm+1 less than  . . .  less than TxN, the crystallization time txm of the m-th recording layer satisfies the relationship tx1 greater than tx2 greater than  . . .  greater than txmxe2x88x921 greater than txm greater than txm+1 greater than  . . .  greater than txN, and the resistance values of the N recording layers in the amorphous phase are different from one another, the resistance values of the N recording layers in the crystalline phase are different from one another, and the sum of the resistance values of the N recording layers is one of 2N values, and the reading apparatus including: an application section through which an electric current pulse is applied to the N recording layers; a resistance measurement device for measuring a sum of the resistances of the N recording layers; and a determination section for determining which of the 2N different values for the sum of resistance values the measured sum of the resistance values of the N recording layers is equal to.
According to still another aspect of the present invention, there is provided a method for writing information in a memory, the memory including: a first recording layer for recording information by utilizing a reversible phase change between a crystalline phase and an amorphous phase which occurs due to increases in temperature caused by application of an electric current pulse: and a second recording layer for recording information by utilizing a reversible phase change between a crystalline phase and an amorphous phase which occurs due to increases in temperature caused by application of an electric current pulse, wherein the crystallization temperature of the first recording layers Tx1, and the crystallization temperature of the second recording layer, Tx2, have the relationship Tx1 less than Tx2, the crystallization time of the first recording layer, tx1 and the crystallization time of the second recording layer, tx2, have the relationship tx1 greater than tx2, and Ra1+Ra2, Ra1+Rc2, Rc1+Ra2, and Rc1+Rc2 are different from one another where the resistance value of the first recording layer in the amorphous phase as Ra1, the resistance value of the first recording layer in the crystalline phase is Rc1, the resistance value of the second recording layer in the amorphous phase is Ra2, and the resistance value of the second recording layer in the crystalline phase is Rc2 and the writing method including steps of: generating at least first to third electric current pulses; and applying the at least first to third electric current pulses to the first recording layer and the second recording layer, wherein, in the step of generating the at least first to third electric current pulses, in order to change the first recording layer from the amorphous phase to the crystalline phase while the phase state of the second recording layer is kept unchanged, the pulse generator generates the first electric current pulse which provides a temperature (T) that satisfies Tx1xe2x89xa6T  less than Tx2 during a time (t) that satisfies tx1xe2x89xa6t, in order to change the second recording layer from the amorphous phase to the crystalline phase while the phase state of the first recording layer is kept unchanged, the pulse generator generates the second electric current pulse which provides a temperature (T) that satisfies Tx2xe2x89xa6T during a time (t) that satisfies tx2xe2x89xa6t less than tx1 and in order to change both the first recording layer and the second recording layer from the crystalline phase to the amorphous phase, the pulse generator generates the third electric current pulse which provides a temperature equal to or higher than the higher one of the melting points of the first and second recording layers.
According to still another aspect of the present invention, there is provided a method for reading information from a memory, the memory including: a first recording layer for recording information by utilizing a reversible phase change between a crystalline phase and an amorphous phase which occurs due to increases in temperature caused by application of an electric current pulse; and a second recording layer for recording information by utilizing a reversible phase change between a crystalline phase and an amorphous phase which occurs due to increases in temperature caused by application of an electric current pulse, wherein the crystallization temperature of the first recording layer, Tx1 and the crystallization temperature of the second recording layer, Tx2, have the relationship Tx1 less than Tx2, the crystallization time of the first recording layer, tx1 and the crystallization time of the second recording layer, tx2, have the relationship tx1 greater than tx2, and Ra1+Ra2, Ra1+Ra2, Rc1+Ra2, and Rc1+Rc2 are different from one another where the resistance value of the first recording layer in the amorphous phase is Ra1 the resistance value of the first recording layer in the crystalline phase is Rc1, the resistance value of the second recording layer in the amorphous phase is Ra2, and the resistance value of the second recording layer in the crystalline phase is Rc2, and the reading method including steps of: applying an electric current pulse to the first recording layer and the second recording layer; measuring a sum of the resistances of the first and second recording layers; and determining which of the four different sums of resistance values, Ra1+Ra2, Ra1+Rc2, Rc1+Ra2, and Rc1+c2, the measured sum of the resistance values of the first and second recording layers is equal to.
According to still another aspect of the present invention, there is provided a method for writing information in a memory, the memory including N recording layers (N is a natural number which satisfies N greater than 2) for recording information by utilizing a reversible phase change between a crystalline phase and an amorphous phase which occurs due to increases in temperature caused by application of an electric current pulse, wherein the crystallization temperature Txm of the m-th recording layer (1xe2x89xa6mxe2x89xa6N) satisfies the relationship Tx1 less than Tx2 less than  . . .  less than Txmxe2x88x921 less than Txm less than Txm+1 less than  . . .  less than TxN, the crystallization time txm of the m-th recording layer satisfies the relationship tx1 greater than tx2 greater than  . . .  greater than txmxe2x88x921 greater than txm greater than txm+1 greater than  . . .  greater than txN, and the resistance values of the N recording layers in the amorphous phase are different from one another, the resistance values of the N recording layers in the crystalline phase are different from one another, and the sum of the resistance values of the N recording layers is one of 2N values, and the writing method including steps of: generating at least N crystallization pulses and amorphization pulse, and applying the at least N crystallization pulses and amorphization pulse to the N :recording layers, wherein, in the step of generating the first to (N+1)th electric current pulses, in order to change only the m-th recording layer from the amorphous phase to the crystalline phase while the phase states of the other recording layers are kept unchanged, the pulse generator generates a crystallization pulse which provides a temperature (T) that satisfies Txmxe2x89xa6Tx less than Tx(m+1)during a time (t) that satisfies txmxe2x89xa6tx less than tx(mxe2x88x921), and in order to change all of the N recording layers from the crystalline phase to the amorphous phase, the pulse generator generates the amorphization pulse which provides a temperature equal to or higher than the highest one of the melting points of the N recording layers.
According to still another aspect of the present invention, there is provided a method for reading information from a memory, the memory including N recording layers (N is a natural number which satisfies N greater than 2) for recording information by utilizing a reversible phase change between a crystalline phase and an amorphous phase which occurs due to increases in temperature caused by application of an electric current pulse, wherein the crystallization temperature Txm of the m-th recording layer (1xe2x89xa6mxe2x89xa6N) satisfies the relationship Tx1 less than Tx2 less than  . . .  less than Txmxe2x88x921 less than Txm less than Txm+1 less than  . . .  less than TxN, the crystallization time txm of the m-th recording layer satisfies the relationship tx1 greater than tx2 greater than  . . .  greater than txmxe2x88x921 greater than txm greater than txm+1 greater than  . . .  greater than txN, and the resistance values of the N recording layers in the amorphous phase are different from one another, the resistance values of the N recording layers in the crystalline phase are different from one another, and the sum of the resistance values of the N recording layers is one of 2N values, and the reading method including steps of: applying an electric current pulse to the N recording layers; measuring a sum of the resistances of the N recording layers; and determining which of the 2N different values for the sum of resistance values the measured sum of the resistance values of the N recording layers is equal to.
Thus, the invention described herein makes possible the advantages of providing: a phase-change memory which stores multi-value information and in which writing and reading of information can be readily performed: a writing apparatus for writing information in such a phase-change memory; a reading apparatus for reading information from such a phase-change memory, and writing and reading methods employed in conjunction with such a phase-change memory.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.